Surface Water flow and the origin of rivers

Surface water flow and origin of rivers

• Rain falling down on land flows down the slope as sheet flow, rill flow and gully flow all of which contribute to stream discharge.
• Underground water oozes at certain points called springs and also contributes stream discharge.

Sheet flow

Sheet flow. Image credit badwaterjournal.com

• It is a type of overland flow or downslope movement of water which takes the form of a thin, continuous film over relatively smooth soil or rock surfaces
• is generated when rain falling onto the earth’s surface flows over the whole surface as a thin layer of water.
• It commonly occurs at the head of the watershed where the slope is gentle and the surface flat e.g. artificial surfaces, rocks etc.

Rill flow

Rill Flow. Image credit MediaWiki.

• Rills are shallow channels (no more than a few tens of centimetres deep) cut into soil by the erosive action of flowing water.
• As the slope steepens,the amount of water increases and sheet flow encounters surface irregularities sheet flow turns into small shallow channels or rivulets known as rills.

Gulley flow

A gulley. Image credit dailymail.co.uk

• Rills in turn join up with other rills and form gullies.
• A gully is a landform created by running water, eroding sharply into soil, typically on a hillside.
• Gullies resemble large ditches or small valleys, but are metres to tens of metres in depth and width.
• The process by which gullies are formed is called gullying.
• A gully may grow in length by means of headward erosion at a knickpoint.
• Gullies are sometimes known as dongas.
• Gullies empty into streams which are perennial rivers.

The results of water erosion

• Sheet flow results in sheet erosion
• This results in the washing away of fertile top soils and shallow soils.
• Rock surfaces and plant roots are also exposed by sheet wash.
• Rill flow results in rill erosion.
• Gully flow results in gullies also known as dongas.
• Both Rill and gully erosions results in the formation of dongas and ravines.

The problems of dongas.

• Can lead to some areas becoming inaccessible as they are difficult to cross especially when it comes to carts and motor vehicles.
• Disrupts communication lines such as roads.
• Reduces the area available for crops pastures and settlements.
• Can lead to the uprooting of trees.
• Contribute to siltation.
• Humans and animals can fall into these ravines leading to injuries.

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